A judge rejected a former US Army soldier's surprise sentencing-day request for a maximum 40-year prison term for trying to help the Islamic State group kill American troops, giving him 14 years behind bars instead.

Cole Bridges, 24, of Stow, Ohio, was sentenced on Friday (local time) after a nearly five-hour Manhattan federal court proceeding in which Bridges, a prosecutor and two of his former commanders told Judge Lewis J. Liman he should get the longest possible prison stint.

"Honestly, I do believe that I deserve the maximum sentence," Bridges, who joined the Army in September 2019, told Liman.

"I know what I did was wrong," he said, adding he would carry "regret for as long as I live."

Liman cited numerous facts that he said demonstrated Bridges was "not a hardened criminal" and said he had no actual communications with the Islamic State organisation.

Instead, he noted, Bridges communicated with an FBI agent posing as a supporter of the terrorist organisation before he was arrested in January 2021 at Fort Stewart, Georgia, where his Army unit — the Third Infantry Division — was assembling after a break from overseas training.

Liman said the sentence would deter other members of the armed forces who might want to attack the military. He said Bridges had "shown signs of remorse," including expressing relief after his arrest that he had been dealing with the FBI rather than terrorists.

Bridges, the judge added, also had not sought any materials from other soldiers that might be useful to the Islamic State organisation. He said the "most chilling evidence" was Bridges' willingness to provide the undercover agent with advice on how the terrorist group could minimise casualties in an attack.

Still, Liman said, Bridges was not the same as Americans who have been criminally charged after travelling to places where the Islamic State group operates and actively assisting terrorists.

After the sentencing, US Attorney Damian Williams said in a statement that Bridges had used his US Army training to pursue a "horrifying goal: the murder of his fellow service members in a carefully plotted ambush."

Bridges pleaded guilty last year to providing material support to the Islamic State organisation, and his attorney, Sabrina Shroff, asked that he be sentenced to the nearly four years he has already served behind bars.

Shroff argued for leniency because Bridges was lured into the plot by undercover US law enforcement agents who posed as supporters of the Islamic State group. She said Bridges was a vulnerable target who was seeking a sense of community after becoming isolated from his family and suffering from depression.

Master Sergeant Greg Fallen, in full military uniform, fought back tears as he described how the arrest of Bridges had destroyed the winning culture of his platoon, leaving everyone "with a sense of defeat."

He said soldiers who had befriended Bridges needed psychological counselling to cope.

"I still can't sleep some nights," Fallen said. "We will suffer with mental anguish for the rest of our lives."

Captain Scott Harper said he was one of three officers aware of the investigation, leaving him to wonder each day if "today was the day he was going to snap."

"My platoon, which could do anything, was instantly destroyed," he said of the fallout after Bridges' arrest. "He betrayed everything he was supposed to stand for."

Assistant US Attorney Sam Adelsberg told the judge that Bridges "attempted to murder American soldiers."

"Cole Bridges is a traitor," he said.

Bridges was largely stoic throughout the sentencing until his father spoke candidly about the "rocky relationship" he had with his son after he got divorced.

"He felt abandoned by me," Chris Bridges, a 25-year Army veteran, said as he and his son wiped their tears.

The father said his "heart goes out" to all the soldiers in his son's unit traumatised by what happened. But he pledged to be there when his son walks out of prison.

"I love him dearly and I'll always be here for him," he said.